The present invention relates to an exhaust gas cleaner for effectively removing nitrogen oxides by reduction from an exhaust gas containing nitrogen oxides and an excess amount of oxygen, and a method for cleaning an exhaust gas with such an exhaust gas cleaner.
Various exhaust gases discharged from internal combustion engines such as automobile engines, etc., combustion apparatuses installed in factories, home fun heaters, etc. contain nitrogen oxides such as nitrogen monoxide and nitrogen dioxide together with an excess amount of oxygen. The term "containing an excess amount of oxygen" means that the oxygen content is larger than the stoichiometric oxygen amount for completely oxidizing or burning unburned or partially burned components such as carbon monoxide, hydrogen, hydrocarbons in the exhaust gas to carbon dioxide and water. The term "nitrogen oxides" (NOx) means nitrogen monoxide and/or nitrogen dioxide.
The nitrogen oxides are one of causes of acid rain, posing a serious problem of environmental pollution. For these reasons, various methods have been proposed to remove nitrogen oxides from exhaust gases emitted from various combustion equipment.
In the case of large, stationary combustion apparatuses such as large combustion apparatuses of factories, ammonia is introduced into an exhaust gas, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced (selective catalytic reduction).
However, such a method is disadvantageous, because ammonia is expensive, because ammonia is so toxic that the amount of ammonia should be controlled by measuring the concentration of nitrogen oxides in the exhaust gas, and because this reduction system generally needs large apparatuses.
There is an alternative method for reducing NOx, which comprises contacting an exhaust gas containing oxygen and NOx with a gaseous reducing agent such as hydrogen, carbon monoxide and hydrocarbons (non-selective catalytic reduction). However, this method requires a larger amount of the reducing agent than its stoichiometric amount relative to oxygen in the exhaust gas to carry out effective removal of NOx. Accordingly, this method is effective only for the exhaust gas having a relatively low oxygen concentration, which is generated by burning nearly at a theoretical air-fuel ratio.
There have been proposed methods of reducing nitrogen oxides by adding to an exhaust gas a reducing agent in a smaller amount than a stoichiometric amount relative to oxygen in the exhaust gas, in the presence of a catalyst such as zeolite with or without carrying a transition metal (Japanese Patent Laid-Open Nos. 63-100919, 63-283727 and 1-130735). Further, Japanese Patent Laid-Open No. 4-354536 has disclosed a method in which nitrogen oxides are reduced on a catalyst comprising .gamma.-alumina carrying alkaline earth metal and/or silver by adding to an exhaust gas a gaseous hydrocarbon.
However, these methods are effective only in a narrow temperature range, in particular, not practical at an exhaust gas temperature lower than about 400.degree. C. Also, their efficiency of removing nitrogen oxides is extremely low in the case of an actual exhaust gas because it contains moisture and sulfur dioxide.